Overrunning is the term used for when an internal combustion engine has a higher rpm than would correspond to the position of the throttle valve, in an Otto engine, or to the injected fuel quantity in a Diesel engine. The simplest special case of overrunning exists when the driving pedal is in the position of rest but the rpm is above a specific value. In the overrunning condition, it is not desirable for the engine to perform work. To this end, the metered quantity of fuel is reduced and the instant of ignition is retarded as needed.
With a view to saving fuel, which becomes more and more important with time, devices were already developed quite early for shutting off the supply of fuel during the overrunning phase. However, a certain cooling down of the engine must then be accepted, and associated with it a worsening of the exhaust emissions for a certain period of time following the end of overrunning operation, and under some circumstances a certain sacrifice in driving comfort during the transition from overrunning to normal operation.
When effecting a cutoff of fuel supply during overrunning, it must be assured that the engine does not drop below a specific rpm value and die, as a result of the compulsory decrease in rpm which then occurs. Appropriate circuitry in many embodiments is already known for both carburetor and injection systems.
In practical operation it has proved difficult to effect a precise setting of the overrunning cutoff; the known systems, therefore, are not without problems under certain operating conditions, such as when the engine is cold.